Ecological communities are structured by interacting processes operating at both local and regional scales, as described in metacommunity frameworks. Pelagic assemblages are often considered as being well mixed and unstructured, so that only few plankton studies describe patch structures in natural ecosystems.  Changes in the phytoplankton community were analyzed in a highly dynamic, open system structured by tidal currents (Wadden Sea, North Sea, Germany). Our study is unique in considering two dimensions of spatial variation - horizontal between sampling sites and vertical with depth - as well as considering temporal variation within sampling sites. Additionally, we coupled changes in the phytoplankton (meta-)community structure to corresponding environmental variables, which were measured while sampling and were complemented by continuous recorded data from a time-series-station. Sampling was conducted in a biweekly rhythm in the backbarrier tidal flat of the Wadden Sea (near Spiekeroog, German Bight, North Sea) in 2009 always around half tide. Sampling was done at three stations characterized by the same depth but different current velocities and sediment grain sizes. Water samples were taken at three different depths. Additionally, benthos samples were taken from the sediment surface.

Results/Conclusions

The results showed that differences in community composition can be attributed to environmental heterogeneity which is coupled with temporal and spatial patterns. The community showed a distinct seasonal pattern: a spring and an autumn diatom bloom were observed, whereas dinoflagellates prevailed during summer. The invasive diatom Mediopyxis helysia, which was first observed in this region, dominated the phytoplankton community throughout the year. High current velocities during half tide promoted high abundances of living benthic diatoms resuspended into the water column. The turnover in vertical and horizontal spatial distance was directly compared to the turnover in time to characterize the degree of patch formation and persistence. Moreover, variance partitioning was used to differentiate between turnover across spatio-temporal versus environmental distance. In conclusion, phytoplankton communities showed a dynamic turnover pattern which could not be separately attributed to “regional” or “local” scales. Spatial and temporal phytoplankton dynamics thus do not conform to one single metacommunity model, but require unification of differently scaled processes.